1. Field of the Invention
This invention relates to reducing methane production in Fischer-Tropsch hydrocarbon synthesis reactions by adding olefins to the reactor feed. More particularly, this invention relates to reducing methane production in catalytic Fischer-Tropsch reactions wherein hydrocarbons are synthesized from a feed comprising a mixture of CO and H.sub.2, by adding one or more olefins to the feed.
2. Background of the Invention
The production of hydrocarbons from mixtures of H.sub.2 and CO via the Fischer-Tropsch process is well-known to those skilled in the art. As opposed to the well-known "methanization" process which produces methane as synthetic natural gas from mixtures of H.sub.2 and CO, the Fischer-Tropsch process is more generally aimed at producing higher value products such as chemical feedstocks and liquid fuels. Thus, high methane make is undesirable in Fischer-Tropsch synthesis processes because it is a relatively low value product which is formed at the expense of more desirable products. It is also uneconomical to try to convert the so-formed methane back into a CO and H.sub.2 mixture and recycle it back into the reactor.
Methane make in Fischer-Tropsch reactions is often expressed by a term known as methane selectivity. Methane selectivity can be defined by either of two methods. They are, (a) mole % CH.sub.4 produced based on the amount of CO consumed or (b) weight % of CH.sub.4 produced based on total hydrocarbon products formed.
Many different catalysts and processes have been disclosed for Fischer-Tropsch synthesis, some of which have extremely high methane make. Thus, U.S. Pat. No. 4,077,995 discloses synthesis of C.sub.1 -C.sub.4 aliphatic hydrocarbons over a catalyst comprising a sulfided mixture of CoO, Al.sub.2 O.sub.3 and ZnO while U.S. Pat. No. 4,039,302 discloses C.sub.1 -C.sub.3 hydrocarbon production using a mixture of the oxides of Co, Al, Zn and Mo. U.S. Pat. No. 4,151,190 discloses producing C.sub.2 -C.sub.4 hydrocarbons from mixtures of CO and H.sub.2 using a supported catalyst comprising a metal oxide or sulfide of Mo, W, Re, Ru, Ni or Pt plus an alkali or alkaline earth metal, with Mo-K on carbon being preferred. U.S. Pat. Nos. 4,243,553 and 4,243,554 disclose MoS.sub.2 as a Fischer-Tropsch catalyst. Many other catalysts are known to be useful for Fischer-Tropsch synthesis employing metals such as iron, copper, titania, etc. These are known to those skilled in the art.
The type of catalyst used and process conditions employed have an important bearing on CH.sub.4 selectivity. For example, nickel gives a high CH.sub.4 selectivity and is used mainly as a methanization catalyst. Methane selectivity usually increases with increasing temperature, decreasing pressure and increasing the H.sub.2 /CO ratio of the feed. Accordingly, process conditions are selected so as to minimize CH.sub.4 selectivity while maintaining a relatively high reaction rate as is well known to those skilled in the art.
It is known that CH.sub.4 selectivity is influenced by the choice of promoter and support, such as alkali metal promotersreducing CH.sub.4 selectivities of iron catalysts. It is also known in the art that noble metals such as ruthenium supported on inorganic refractory oxide supports exhibit superior hydrocarbon synthesis characteristics with relatively low methane production. Thus, U.S. Pat. No. 4,088,671 suggests minimizing methane production by using a small amount of Ru on a cobalt catalyst. Examples of supported ruthenium catalysts suitable for hydrocarbon synthesis via Fischer-Tropsch reactions are disclosed in U.S. Pat. Nos. 4,042,614 and 4,171,320 the disclosures of which are incorporated herein by reference. It is also known that the type of support used also influences methane production. In the case of supported ruthenium catalysts, the use of a titania or titania containing support will result in lower methane production than, for example a silica, alumina or manganese oxide support.
Those skilled in the art recognize the need for reducing methane production still further, even when employing catalysts comprising ruthenium supported on titania.